The present invention generally relates to methods and systems for transmitting data in conformance with the high-level data link control (HDLC) procedure (hereinafter simply referred to as HDLC data), and more particularly to a method and a system for transmitting the HDLC data via an asynchronous transfer mode (ATM) network.
Recently, there is active research on the use of the ATM network as a broadband integrated services digital network (ISDN). Various kinds of information including voice and data may be transmitted via the ATM network. On the other hand, the data transmission employing the HDLC procedure prescribed by the CCITT Recommendations is popular, and it is desirable that the HDLC data can be transmitted via the ATM network with a satisfactory efficiency.
The HDLC procedure is a flag synchronization transmission control procedure. In other words, information having an arbitrary bit length is regarded as a transfer unit called a frame, and the HDLC procedure enables transfer of continuous information in frames.
FIG. 1 shows the structure of a HDLC frame, and FIG. 2 shows an example of a transfer of the HDLC data.
In the HDLC frame shown in FIG. 1, one frame is made up of a flag F having a bit structure "01111110", a 1-byte address field A, a 1-byte control field C, an n-byte information field I (n is an arbitrary integer), a 2-byte frame check sequence FCS, and a flag F.
As shown in FIG. 2, the flags F are transferred between the transfers of the HDLC frames HDLC1 through HDLC4 for synchronization purposes. In addition, in order to prevent a bit pattern identical to that of the flag F from occurring within the frame, a bit "0" is forcibly inserted after 5 consecutive "1"s. The inserted "0" is deleted at a receiving apparatus to restore the original data. Furthermore, according to the HDLC procedure, there is a rule to transmit a predetermined number (7 or more) consecutive "1"s if aborting the data transmission.
On the other hand, the conventional circuit switching system (that is, the synchronous transfer mode or STM) sets a path by fixedly assigning the band to be used. For this reason, the band utilization efficiency is poor in that the band is unused when the path is not used. But in the case of the ATM, digital data are sectioned into cells which have a fixed length, and the cells are transmitted only when the information is generated. Each cell is made up of a 5-octet header and a 48-octet information field, that is, a total of 53 octets.
FIG. 3 shows the structure of the ATM cell. As shown, one ATM cell is made up of 53 octets (bytes) in total, and the first 5 octets form the ATM header while the remaining 48 octets form the information field. The ATM header is used to transfer information including a virtual path identifier VPI, a virtual channel identifier VCI and head error check HEC. The 48-octet information field is provided with a 2-octet adaptation header and a 2-octet adaptation trailer which include information related to control, in addition to the data field.
When transmitting the above described HDLC data via the ATM network, the frame data and the flags between the two successive frames will have to be disassembled into ATM cells before inputting the same to the ATM network. When outputting the ATM cells from the ATM network, the ATM cells will have to be assembled and converted back to the original HDLC data.
However, in the HDLC data, the flags and the "0" inserted after five consecutive "1"s are not real data. Therefore, there is a problem in that information other than the real data is transmitted if the HDLC data is simply disassembled into the ATM cells and input to the ATM network, and there is a demand to improve the data transmission efficiency.